Currently, the range of commercial and industrial products obtainable from photoautotrophic microorganisms is broadened with the help of genetic engineering. In the following, the term photoautotrophic microorganisms will be used for cyanobacteria and eukaryotic micro-algae of different divisions, mainly rhodophyta, heterokonta, euglenophyta and chlorophyta, with a size of the cells or cell aggregates below 20 μm.
The genomes of the cyanobacterium Sychenocystis sp. PCC 6803, the green alga Chlamydomonas reinhardtii and other biotechnologically important photoautotrophic microorganisms have been decrypted, and transgenic strains with the potential for producing different biotechnologically relevant organic substances by photosynthesis became available. The main source of carbon in photo-bioreactors is carbon dioxide supplied to cell suspensions of these microorganisms by introducing air or a carbon dioxide-air mixture into the cell suspension. The avoidance of carbon dioxide depletion zones and light deficiency in photo-bioreactors is technically difficult when cyanobacteria and micro-algae occur in a high biomass concentration. The use of high concentrations of potassium or sodium bicarbonate is a well-known way of increasing the pH and promoting the growth of the culture of the alkaliphilic large-cell organism Spirulina plantenis in illuminated open tanks exchanging carbon dioxide with the atmosphere.
From stoichiometric reasons a bicarbonate medium used as the sole carbon source cannot contain enough stock for the growth up to high biomass concentrations, unless its carbon concentration is more than 200 mmol L−1. It is, however, known that many cultures die off in media with much lower bicarbonate concentrations. The growth of shaken cultures of photoautotrophic microorganisms can be promoted by a membrane-bordered carbon reservoir (DE 2009/10 2008 029 169). The latter consists of a concentrated aqueous buffer solution of bicarbonate and carbonate ions in a bag made of a thin polyethylene film. Carbon dioxide is discharged with controlled rate to the cell suspension through the gas-permeable membrane of this bag.